The expenses of paintball can add up quickly!  Constantly re-filling air tanks, paintballs, paintball field/park fees make regular paintballing a bit expensive at times.  CO2 tank O-rings are an essential piece of gear; without them, there is no seal between your tank and paintball gun and air will leak from your tank.  If you aren't careful, you can go through o-rings like candy and this can add up expenses over time.  O-rings are generally not expensive but if you have to go through a lot of them, it can add up to a lot of money if you play a lot.